slip factor??

[capnzee]

Looking for thoughts. I have a Sleekcraft sst weighing 1300 lbs plus load. At 6500 rpm (1.87:1 @ prop) my speed is 73 mph. I calculate this to be about 20% slip with a 28" X 141/2" Yamaha copy (drag) propellor. If my calculations are correct, is this what I should expect? I was hoping to get better efficiency. Do I need a wheel with more blade area? and will this improve efficiency? On lighter boats my slip has been less or near 10% (inboard hydro) Capnzee

[Raceman]

It could be just that one particular prop, or the fact that the design isn't the best for your boat, as well as setup issues, such as setback or transom height.

We seem to always use 10% as an estimated rule of thumb, but I suspect that in the real world a lot of boats slip more than that.

[outboards4life]

A chopper prop with the weed cutters removed and blended in with some cup is a great prop for an SST. Thats what I ran on mine when I had it. with a 26 I got it up to 84 mph. Ran wonderful. its a heavy boat that needs a prop to pick the bow up. Thats where the chopper works great.
Travis:cool:

[capnzee]

Raceman, the prop-shaft is currently 2 1/4 inches below the center pad of the mod v-p type boat. The engine is a 200 fishing Black Max. I have tried 5 props ranging from 24" to 28" in pitch and can't seem to improve on the efficiency. I have tried cleavers and elephant ears but have not had a large area blade on the engine. Based on this thread I will plan my next move. I had thought that 10% slip was a little optimistic, but 20% slip doesn't cut it. Thanks for the reply, looking for more comments! capnzee

[capnzee]

Got that last thread while I was submitting my last reply.
My goal was to get 80 mph out of the Sleek and you have run 84. That is super news to me. What rpm did you turn to get that kind of speed and who made your prop? capnzee

[outboards4life]

I was turning a little over 7000 rpm at that speed. I'm sure I could have turned a 28 on it at about 6800 maybe slightly less. The prop was just a merc chopper. that I had the weed cutters hacked off and some cup put in it by a local shop here. Randy (Aquaholic) does some nice work with choppers and usually sells them at a very fair price. Get a hold of him.
Travis
:cool:

[rude tim]

I would try a 26 pitch et. I have found that they carry the bow better than a yamaha drag prop, you should use less trim and have less slip.

[Techno]

Your tach might be off. Wondering if the prop is too deep?

[capnzee]

Good thoughts; I have tried several tachs with the same results. I figure the prop is running deeper than desired (2 1/4" prop shaft to bottom) but have hesitated in raising it any higher until I can rebuild my set back. I'm at the top plus about 4 inches at this time. ( The Sleekcraft has a very low transom). I have a modified 2.5 powerhead coming on a off shore midsection with a Cle unit on the bottom. I would like to get the Sleek craft running a little faster then hopefully sell the entire rig and get an STV! Based on what I have heard so far, I am going to try a 26" to 28" chopper next. I may try to get the prop shaft up a little closer to the surface also. Thanks for all of the interest and help! Capnzee

[racer]

If you are running possitive trim with a cle unit on a sleek it will slip, you need a prop with bow lift and a lower that is not as trim/height critical.

[stvhelm]

It sounds like you dont have enough setback. If you need alot of positive trim to get your top speed then you need to make some changes. some of those sleeks dont have a notched transom either. if yours dont have a notch then you should have at least ten inches bolted on. you may want to consider some reinforcements too. you shouldnt have to use more than 1 or 2 degrees positive trim to get your best top speed.

[capnzee]

My 1984 Sleek does not have a notched transom and I do have 10 inches of setback. The 2.4 Black max that is currently on the Sleek is a fishing engine with a standard 1.87 L/U. Again, the boat weighs 1300 lbs plus payload. After listening to all, I think my problem is in excess slippage at the prop and the answer may be a prop with more blade area and a lot of bow lift built into the cup. I may be asking for more than a 2.4 Black Max 200 can give me, but I am not ready to quit just yet! Capnzee